1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device.
In this specification, a semiconductor device means all types of devices that can function by utilizing semiconductor characteristics, and an electro-optical device and a semiconductor integrated circuit are semiconductor devices.
2. Description of the Related Art
A transistor whose channel region includes a semiconductor thin film over an insulating surface is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to the transistor. In recent years, attention has been drawn to a technique by which a transistor whose channel region includes an oxide semiconductor instead of silicon to be applied to an electronic device or an optical device. For example, Patent Document 1 and Patent Document 2 disclose a technique by which a transistor is manufactured using zinc oxide or an In—Ga—Zn-based oxide as an oxide semiconductor to be used as a switching element of pixel or the like of an image display device.
It has been pointed out that by entry of impurities such as hydrogen, an electrically shallow donor level is formed and electrons to be carriers are generated in an oxide semiconductor. As a result, the threshold voltage of a transistor including an oxide semiconductor is shifted in the negative direction and the transistor becomes normally-on, so that leakage current in a state where voltage is not applied to the gate (that is, in the off state) is increased. Thus, the entry of hydrogen into an oxide semiconductor film is suppressed by providing an aluminum oxide film having a property of blocking hydrogen over the entire region of a substrate so as to cover a channel region in the oxide semiconductor film, a source electrode, and a drain electrode, so that generation of leakage current is suppressed (see Patent Document 3).